Hey guys,
is there any chance to create an abstract constructor like:
abstract class ABC {
abstract this();
}
DMD always says "...this non-virtual functions cannot be abstract" -
when I use an interface like:
interface ABC {
this();
}
I get a similar error: "...constructors, destructors, postblits,
invariants, unittests, new and delete functions are not allowed in
interface ABC"
Is there any solution or is it possible to create such inheritances
in DMD?

Hey guys,
is there any chance to create an abstract constructor like:
abstract class ABC {
abstract this();
}
DMD always says "...this non-virtual functions cannot be abstract" -
when I use an interface like:
interface ABC {
this();
}
I get a similar error: "...constructors, destructors, postblits,
invariants, unittests, new and delete functions are not allowed in
interface ABC"
Is there any solution or is it possible to create such inheritances
in DMD?

Hey guys,
is there any chance to create an abstract constructor like:
abstract class ABC {
abstract this();
}
DMD always says "...this non-virtual functions cannot be abstract" -
when I use an interface like:
interface ABC {
this();
}
I get a similar error: "...constructors, destructors, postblits,
invariants, unittests, new and delete functions are not allowed in
interface ABC"
Is there any solution or is it possible to create such inheritances
in DMD?

An abstract constructor makes no sense. It also makes no sense for an
interface to have a constructor. A constructor is specific to the class that
it's on. It is not virtual at all and has nothing to do with the inheritance
hierarchy beyond the fact that subclasses can and will call the constructors
of base classes in their own constructors.
- Jonathan M Davis

Hey guys,
is there any chance to create an abstract constructor like:
abstract class ABC {
abstract this();
}
DMD always says "...this non-virtual functions cannot be abstract" -
when I use an interface like:
interface ABC {
this();
}
I get a similar error: "...constructors, destructors, postblits,
invariants, unittests, new and delete functions are not allowed in
interface ABC"
Is there any solution or is it possible to create such inheritances
in DMD?

I think what you are trying to do is say, "if a class implements interface
ABC, it must have a default constructor". Such a requirement is faulty.
The point of an interface is to able to pass a portion of a class'
functionality to a function during runtime. However, the instance must
*already exist*. It makes no sense to posit requirements on the
constructor.
What you want is a compile-time requirement using a template constraint.
You may think "damn, but I don't want to make my function a template", I'd
say see previous point ;)
-Steve

when I use an interface like:
interface ABC {
this();
}
I get a similar error: "...constructors, destructors, postblits,
invariants, unittests, new and delete functions are not allowed in
interface ABC"
Is there any solution or is it possible to create such

inheritances

in DMD?

I think what you are trying to do is say, "if a class implements

interface

ABC, it must have a default constructor". Such a requirement is

faulty.

The point of an interface is to able to pass a portion of a class'
functionality to a function during runtime. However, the instance

must

*already exist*. It makes no sense to posit requirements on the
constructor.
What you want is a compile-time requirement using a template

constraint.

You may think "damn, but I don't want to make my function a

template", I'd

say see previous point ;)
-Steve

Okay, thanks... perhaps someone know a better solution: I have one
big file which contains some other files (let's say: blocks). Each
block has it's own signature... by reading the big file, I read the
signature of each block. Based on the signature, I read block A,
block B or another Block. To do that, I want call the block-specific
constructor which reads the next bytes.
!Semicode:
...
ABC[] blocks;
...
while (!eof(bigfile)) {
read(signature);
if (signature==A) blocks ~= new A(bigfile);
else if (signature==B) blocks ~= new B(bigfile);
...
}
...

when I use an interface like:
interface ABC {
this();
}
I get a similar error: "...constructors, destructors, postblits,
invariants, unittests, new and delete functions are not allowed in
interface ABC"
Is there any solution or is it possible to create such

inheritances

in DMD?

I think what you are trying to do is say, "if a class implements

interface

ABC, it must have a default constructor". Such a requirement is

faulty.

The point of an interface is to able to pass a portion of a class'
functionality to a function during runtime. However, the instance

must

*already exist*. It makes no sense to posit requirements on the
constructor.
What you want is a compile-time requirement using a template

constraint.

You may think "damn, but I don't want to make my function a

template", I'd

say see previous point ;)
-Steve

Okay, thanks... perhaps someone know a better solution: I have one
big file which contains some other files (let's say: blocks). Each
block has it's own signature... by reading the big file, I read the
signature of each block. Based on the signature, I read block A,
block B or another Block. To do that, I want call the block-specific
constructor which reads the next bytes.
!Semicode:
...
ABC[] blocks;
...
while (!eof(bigfile)) {
read(signature);
if (signature==A) blocks ~= new A(bigfile);
else if (signature==B) blocks ~= new B(bigfile);
...
}
...

No special requirements are necessary. How would this compile if A or B
did not have a bigfile constructor? The interface specification is not
needed.
If D supported runtime reflection (and it does to a very very small
degree), then you could use it to ensure the correct constructor is
available.
-Steve

First, this should work. Second, there is no good reason to do this. An
abstract class is allowed to have an empty constructor, but does not force
one to implement a Stream-accepting constructor.
For example, this would be a valid subclass:
class A : ABC {
this() { super(null); }
}
I'm not very good at explaining things sometimes, but if you think about
this for a while, it should sink in. Construction is a concrete activity,
you need to have all information about a class to construct it.
Interfaces and polymorphism is an abstract activity, you do not need to
have the full definition of the class to call virtual methods.
There is no point to have a constructor definition to force a signature,
because it just won't matter. The derived class does not have to override
base constructors because base constructors are not virtual methods.
What you probably want is a factory method such as:
ABC abcfactory(Stream s)
{
header = readHeader(s);
if(header == Aheader)
return new A(s);
...
else
throw new Exception("Unknown header");
}
What you should put in ABC is methods you would call on the objects
*after* they are constructed.
Now, with runtime reflection, you can "look up" classes based on the class
names and their appropriate constructors. I've written such systems in
C#, and it's not extremely easy to do it safely. It's much more
straightforward and less error prone to just use a factory method like the
above.
-Steve